The present invention relates generally to pumps, and, more specifically, to blood pumps.
U.S. Pat. No. 5,924,975 discloses a linear pump which is implantable as a left ventricular assist device (LVAD) that assists a damaged heart in pumping blood. This pump includes a tubular piston disposed coaxially inside a tubular housing. Check valves are joined to the housing and piston for effecting unidirectional fluid flow as the piston reciprocates in the housing.
The piston is driven by a linear motor that includes axial drive coils mounted inside the housing which cooperate with a pair of magnet rings mounted in opposite ends of the piston. A magnetic circuit is thusly formed between the piston and housing, and the coils are commutated to control axial oscillation of the piston and resulting pumping therefrom.
The piston includes a cylindrical outer surface or journal which is spaced radially inwardly from the housing bore to define a hydrodynamic bearing having a radial gap which receives a portion of the blood being pumped. The surfaces of the piston end housing bore are smooth and define a nominally circular annulus in which the journal bearing is effected.
In a preferred embodiment, the piston rotates in addition to axially oscillating for developing hydrodynamic pressure in the blood for supporting the piston and preventing contact with the housing to prevent blood damage. The piston may be rotated by various methods which vary in complexity and efficiency.
For example, the two magnet rings used in the axial motor may be modified for use also in a rotary motor. The rings may be configured to have circumferentially spaced apart zones of different magnetic field flux density which effectively form unidirectional magnetic poles around the piston. The magnet rings are thusly effective for magnetically cooperating with the axial drive coils for axial oscillation, while also being effective for magnetically cooperating with a rotary drive coil for spinning the piston during operation.
However, this integrated axial and rotary motor configuration compromises performance of the axial motor and the rotary motor, and has correspondingly reduced efficiency. In an exemplary configuration for use in a LVAD pump, the resulting inefficiency of the motor requires about 5 watts of rotary power for developing a suitable hydrodynamic bearing. Since the rotary poles formed in the magnet rings reduce the average radial magnetic field therefrom, power for the linear motor must correspondingly increase from about 6 watts to about 7 watts in a practical example.
Motor efficiency is a significant factor in blood pump design because it affects pump size and power requirements. The pump should be as small as practical for being implanted in a living body, and should have minimum power consumption for reducing battery requirements.
Accordingly, it is desired to provide an improved linear pump having a new combination of linear and rotary motors.